Proto-oncogenes participate in a complex web of growth promoting and inhibitory signaling circuits involved in controlling normal cellular proliferation, differentiation and functional responses. They have been found to be constitutively activated in a retroviral context by a variety of mechanisms such as point mutation, truncation and insertional mutagenesis. Somatic mutation, chromosomal translocation, amplification and/or overexpression are the main mechanisms by with proto-oncogenes are activated in human tumors. Recent work has elucidated specific roles for various cellular and viral oncoproteins in different signal transduction cascades. This symposium is structured to provide the latest developments in understanding the positioning and function of these proto-oncogene products in the context of individual and interconnecting signaling networks and to define which oncogene networks are most relevant in human cancer. Emphasis will be placed on cell surface receptors, integrins, adaptor proteins, intracellular tyrosine and serine/threonine kinases, phosphatases, Ras, Rac and Rho family members, guanine nucleotide exchange factors, the MAP kinase family, effectors of phospholipid metabolism, and transcription factors.